Large generators and motors are routinely examined for laminate shorts when stationary. Various methods are available for this purpose.
One of these methods comprises the magnetization of the entire laminated body by means of an auxiliary coil at the mains frequency, and the measurement of stray fields on the inner surface of the stator bore. The magnetization is carried out to relatively low values of the magnetic induction, typically to about 10% of the normal operating induction. This method of measurement is also known by the name “low-induction laminate short measurement” or by the name “ELCID” (electromagnetic core imperfection detector).
By way of example, U.S. Pat. No. 4,996,486 describes one method of this type. The prior art is therefore for the laminated stator core to be magnetized by means of an auxiliary coil and a sinusoidal auxiliary voltage applied thereto at the mains frequency, to about one tenth of the operating induction. This auxiliary voltage is normally derived directly from the mains voltage. An electrical recording coil is then moved away from the surface of the stator bore, with the recording coil being located close to the surface of the laminated core.
The currents which flow as a result of the interlaminar short circuits in the laminated core now induce voltages with a characteristic phase angle and amplitude magnitude in the recording coil. The characteristic phase angles and amplitudes make it possible to distinguish between points where there are laminate-short currents and points where there are no laminate-short currents. It is therefore possible to locate laminate shorts, and to assess the magnitude of the short-circuit currents, by means of this stray-field recording coil.
Furthermore, patent specification U.S. Pat. No. 6,903,556 proposes a system which operates with magnetization voltages at a frequency higher than the mains frequency.
The known methods have the disadvantage that it is often difficult to interpret the measurement results since the voltages induced by the laminate-short currents are generally very small. In particular, strong stray fields of the stator main field or losses in additional fields, which occur, for example, as a result of currents induced in short-circuited conductor loops can conceal the effect of the actual short-circuit current through the defective point, thus making detection more difficult.
This is particularly the case with small laminate defects which cause only small currents and have only a small magnetic effect. Disturbing additional fields occur particularly when testing hydrogenerators when the rotor has not been removed for testing, as a result of which the individual poles exert a magnetic effect.
According to U.S. Pat. No. 6,903,556, the reliability of the measurement can be increased by magnetizing the stator at a higher frequency. A sinusoidal AC voltage at a higher frequency is applied to the magnetization coil for this purpose. According to the “ELCID” measurement method, the so-called “magnetic potential differences” (MPD) are then determined. These are proportional to the respective electric currents which influence the field through the measurement coil. Since only a smaller magnetization current is required at a higher frequency to create the same short-circuit current through the defect point as in the case of magnetization at a lower frequency, the corresponding MPD component of the short-circuit current is greater in relation to the corresponding component of the field current than at a lower frequency, that is to say the measurement is more accurate and more sensitive. This measurement method has the disadvantage of the additional complexity required for the high-frequency generator for the field current. A further disadvantage is that it is necessary to work with precisely selected and known and/or measured values of the stator induction, since the measurement results are compared with absolute limit values and are then assessed.